1. Field of the Invention
The present invention relates to a pumping circuit of a semiconductor device, and more specifically, to a pumping circuit of a semiconductor device in which a first transfer pump and a second transfer pump perform one-step or two-step pumping operation to output a high voltage according to an externally applied voltage.
2. Discussion of Related Art
A high voltage generator is a device for supplying a constant high voltage Vpp to circuits within a chip, which requires a voltage higher than an externally applied voltage Vdd, in semiconductor memory devices. This high voltage generator is implemented by a charge pumping method.
In a dynamic random access memory (DRAM) being a kind of semiconductor memory devices, it is important to maintain a high voltage Vpp being the power source used in a core unit of the device over a predetermined level, and to reduce consumption of current. The high voltage Vpp that uses a level higher than that of an externally applied voltage Vdd obtains the electric potential of 2 to 3 times of the externally applied voltage Vdd through a charge pumping operation. In view of the core unit of the DRAM that is being used, the level of the high voltage Vpp is about 3.3V. It is required that the high voltage Vpp be produced regardless of the externally applied voltage. Therefore, when a low external voltage is applied, 3 times of the externally applied voltage can be obtained through twice pumping operations. For example, if the externally applied voltage Vdd is 1.5V, a pump having efficiency of 100% can pump the voltage up to 3.0V through once charge pumping operation. Thus, in an actual device having efficiency of below 100%, a voltage lower than 3.0V can be implemented. If a low external voltage is applied as such, the voltage is pumped in two steps to implement a target voltage of 3.0V or more.
In this pump, however, if an external applied voltage is over a given level, e.g., 2.5V, a two-step pumping operation is performed although the voltage can be pumped to 5.0V through once charge pumping operation. Thus, there are problems in that efficiency of the pump is lowered and consumption of the current increases.